Photosystem II (PSII) is an integral membrane protein complex in which photochemistry, electron and proton transport and catalysis of water oxidation and quinone reduction are facilitated and regulated by multiple ion cofactors and prosthetic groups, and the protein/lipid environment. The long-term objective of this project is to develop an understanding at the molecular level of the structure and assembly of the tetranuclear Mn active site of water oxidation, the catalytic mechanism of water oxidation, the electron-transfer properties of PSII, and the damaging side reactions that occur in the process of water oxidation. In the proposed studies, electron paramagnetic resonance spectroscopy will be used to study the assembly, coordination and redox chemistry, and the intermediate oxidation states of the tetranuclear Mn complex. These studies will take advantage of a newly-developed class of redox-active herbicides that will be used to